Question

calcule a soma dos 15 primeiros termos da PA⊂-3,4,11, ...⊃.

Answer 1

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1° calcular an

$a_n=?~~~~ a_1=-3~~~~~~~r=4-(-3)=4+3=7~~~~~~~n=15 \\ \\ a_n=a_a+(n-1)r \\ \\ a_n=-3+(15-1)7 \\ \\ a_n=-3+(14).7 \\ \\ a_n=-3+98 \\ \\ a_n=95 \\ -------------- \\ \\ S_n= \frac{(a_1+a_n)n}{2} \\ \\ S_{15}= \frac{(-3+95)15}{2} \\ \\ S_{15}= \frac{(\not92^{46}).15}{\not2} \\ \\ S_{15}=46.15 \\ \\ S_{15}=690$